feat(a11y): label form controls + overlays (P3-4)

Accessible names for ~15 controls that lacked them: invite/join/create-room/
account-data/image-pack/private-note/power-level inputs (visible <label htmlFor>
where a label exists, else aria-label); the two range sliders (night-light
intensity, noise-gate threshold); the soundboard file input; media <video>
elements; and the Media Gallery (region) + Search (dialog) overlays. Hidden
notification/preview <audio> marked aria-hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/image-pack-view/PackMeta.tsx

@@ -200,12 +200,24 @@ export function ImagePackProfileEdit({ meta, onCancel, onSave }: ImagePackProfil
         </Box>
       </Box>
       <Box direction="Inherit" gap="100">
-        <Text size="L400">Name</Text>
-        <Input name="nameInput" defaultValue={meta.name} variant="Secondary" radii="300" required />
+        <Text as="label" htmlFor="image-pack-name" size="L400">
+          Name
+        </Text>
+        <Input
+          id="image-pack-name"
+          name="nameInput"
+          defaultValue={meta.name}
+          variant="Secondary"
+          radii="300"
+          required
+        />
       </Box>
       <Box direction="Inherit" gap="100">
-        <Text size="L400">Attribution</Text>
+        <Text as="label" htmlFor="image-pack-attribution" size="L400">
+          Attribution
+        </Text>
         <TextArea
+          id="image-pack-attribution"
           name="attributionTextArea"
           defaultValue={meta.attribution}
           variant="Secondary"